Log into your DSL modem and check the stats. If your last resync or reestablishment of the PPP connection occurred at the time of or after the dropped call, that is almost certainly your problem. Also, confirm that the S/N margins are decent, at least 10 dB in each direction.
When you changed from 10 Mbps to 3 Mbps service, was anything changed at your home (new modem or router, new wire from the ISP)? If so, please provide details.
Assuming that your service is Google Voice, you might troubleshoot this problem by shutting down the OBi for a few days and using Gmail to make and receive calls. If they still get dropped, you'll know that the OBi is not at fault. Alternatively, you might run a continuous ping to your ISPs first gateway. When the OBi drops a call, check whether there were lots of lost packets.
If no luck, post details about ISP, modem, separate router (if any), service(s) used on OBi, etc.